how to improve eyesight for someone already wearing glasses?

Answer

Wa alaikumus salaam

As ruqyah it is recommended to recite the following 7 or 11 times:

فَكَشَفْنَا عَنْكَ غِطَاءَكَ فَبَصَرُكَ الْيَوْمَ حَدِيدٌ

One may also recite "Ya Noor" then blow on thumbnail and rub it to the eyes.